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Regenerative Braking

April 09th 2020
ENSAK 2019 / 2020
Class : CI / GE / S6
Regenerative Braking in DC Motors
For the motor, this braking method is called regenerative because its works as a generator
and the kinetic energy of the motor is converted into electrical energy and returned to the
supply. For a DC motor, it may go into regenerative braking mode if its speed becomes
sufficiently high so as to make back emf greater than the supply voltage Eb > V. The direction of
the current armature Ia under this condition will reverse imposing torque which is opposite to
the direction of rotation. The normal motor operation is where the armature motoring current
Ia is drawn from the supply and as usual Eb < V since Eb = kφn1. The question is how speed on
its own become large enough to make Eb < V causing regenerative braking. An Example of a
situation where the mechanical load itself becomes active is when a DC motor is coupled to the
wheel of locomotive which is moving along a plain track without any gradient (Figure 1). The
machine is running as a motor at a speed of n1 rpm. However, when the track has a downward
gradient (Figure2), component of gravitational force along the track also appears which will try
to accelerate the motor and then increase its speed to n2 such that Eb= kφ n2 > V. In such a
scenario, direction of Ia reverses, feeding power back to supply. Regenerative braking here will
not stop the motor but will help to arrest rise of dangerously high speed.
Another way to achieve this type of braking is by using a chopper (shown in figure 3).
During regenerative braking mode, the chopper is gated so that the motor armature is short
circuited through the chopper. Kinetic energy of the motor is partly dissipated in armature
resistance and partly stored in inductance La. When chopper is turned off, the freewheeling
diode is turned on and the energy stored in inductance La is returned to the supply source.
• The average voltage of chopper : Vch = (1 – α) V
• Regenerated power : Pg = Vch x Ia = (1 – α) V Ia
• Generated voltage Es : Eg = Vch + IaRa = (l – α) V + IaRa
Figure 3 : Regenerative Braking of separately excited DC Motor
Figure 1 : Machine operates as Motor
Figure 2 : Machine enters Regenerative Mode
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